The invention relates to a method and apparatus for converting PAM analog signals into PCM digital signals and for converting PCM digital signals into PAM analog signals in telecommunication installations having telephone stations. Such a known system is described in commonly assigned U.S. Pat. No. 4,056,820 filed on July 27, 1976, and issued to Hofer on Nov. 1, 1977. The telephone stations each comprise a transmitting unit supplying analog signals and a receiving unit accepting analog signals. The telecommunication installations can be connected to signal receiving units which accept PCM digital signals and signal output units which supply PCM digital signals. As is generally known in the art, the PCM digital signals are in the form of a PCM pulse train having pulses recurring in pulse frames or channel time slots in successive cycles. The telecommunication installation operates for the duration of the pulses of the pulse train, i.e., the channel time slot pulses of the channel time slot pulse train. Associated therewith for the conversion of the analog signals into digital signals is an analog-to-digital converter which is also utilized for receiving a digital signal to be converted into an analog signal. The analog-to-digital converter allocated to the telephone stations works along the iterative principle. The invention employs a counter, an intermediate register controlled by the signals of the counter, a digital-to-analog converter following the intermediate register and a comparator. One input of the comparator is connected to the output of the digital-to-analog converter. Analog signals to be converted into digital signals are applied at another input of the comparator. The comparator is capable of controlling the supply of the counter signals to the intermediate register. The digital signal which is to be converted into an analog signal is intended for the receiving unit of the telephone station. The digital signal is converted during one subinterval of the counter which corresponds to one pulse frame. The analog signal is converted into a digital signal and supplied from the associated transmitting unit of the particular telephone station within another subinterval.
According to the method described hereinabove the counter associated with the analog-to-digital converter is continually operated. Because the analog-to-digital converter is in continuous operation, problems may arise during the conversion of digital signals into analog signals. For example, the counter might operate so fast that the digital-to-analog converter might be activated at the same time another digital signal is already being converted into an analog signal. This could lead to faulty multiple conversions of the digital signals.
It is therefore an object of the invention to provide an easy method for ensuring that a digital signal is converted just once into an analog signal.